Our overall goal is to determine the role of the adenomatous polyposis coli (APC) gene in DNA damage-induced DNA repair activity and breast carcinogenesis. Down-regulation of APC is a predominant phenomenon in breast cancer, but neither its role in this disease nor the molecular alteration(s) underlying carcinogenesis in normal breast epithelial cells have been characterized. In the past, the role of mutated APC has been investigated in various cancers. In our studies, we propose a highly novel concept that it is the decreased level of APC that is responsible for the decreased DNA repair capacity, which is the cellular mechanisms underlying carcinogenesis in normal breast epithelial cells. Our preliminary data indicate that APC protein interacts with base excision repair proteins, including proliferating cell nuclear antigen (PCNA) and apurinic/apydmidinic endonudease (APE). We hypothesize that the normal levels of APC promote formation of an active complex with PCNA and APE that facilitates repair of abasic DNA through a base excision repair pathway thereby preventing accumulation of gene mutations in normal breast epithelial cells. Decreased levels of APC result in inappropriate binding of the PCNA/APE-complex at the abasic DNA, decreased base excision repair activity, accumulation of gene mutations, and transformation of normal breast epithelial cells. To test this hypothesis, we will use human normal breast epithelial cell lines treated with DNA-damaging mammary carcinogens. We will: (1) Determine that the carcinogen exposure attenuates APC gene expression in normal breast epithelial ceils; (2) Characterize the structure-function relationships of APC with PCNA and APE; (3) Utilize a novel base excision repair assay system with functional purified repair complexes to characterize changes in the dynamics of these complexes in carcinogen-treated normal breast epithelial cell lines; and (4) Examine the function of APC as a PCNA recruiting factor onto abasic DNA to regulate base excision repair activity in carcinogen-treated and untreated normal breast epithelial cells. This project will, for the first time, provide a novel paradigm for understanding the molecular basis for APC function in DNA damage-induced base excision repair and the transformation of normal breast epithelial cells and will facilitate development of chemotherapeutics for prevention of breast cancer progression.